1. Field of the Invention
This invention relates to basic circuits in a multivalued logic circuit system, analog circuit system or the like, examples being subtracting circuits, adding circuits, literal circuits and T gates. The invention further relates to switching circuits for constructing these basic circuits, such as, for example, floating threshold switching circuits, floating window switching circuits and threshold SPDT switching circuits.
2. Description of the Prior Art
Extensive research in the field of multivalued logic and associated arithmetic circuits is underway with the aim of compensating for or overcoming the several limitations of two-valued (binary) logic, which is the foundation of many digital circuit systems, the foremost of which is the computer. Whereas two-valued logic deals with the two values 0 and 1 and the signals employed by a two-valued logic circuit system have two levels orresponding to these two values, multivalued logic deals with three or more values and the signals used by a multivalued logic circuit system have three or more levels.
Multivalued logic (and a multivalued logic circuit system) has the following advantages over two-valued logic (and a two-valued logic circuit system):
(1) It is possible to describe an indeterminate state between 0 and 1 (as by employing three values).
(2) The wiring area on an IC substrate and the number of pins can be reduced to enable a higher degree of effective integration. In the case of 64 values, for instance, one sixth the wiring area of a two-valued logic circuit is sufficient.
(3) The realization of a ten-valued (decimal) machine would make it possible to employ logic the same as that used by human beings, so that the encoders and decoders required by two-valued machines would be unnecessary.
The conventional circuit systems can be broken down into two categories from the viewpoint of the circuit modes used by an information processing system, rather than from the viewpoint of whether two or multiple values are employed. The first of these two categories is a voltage mode circuit system, in which information is expressed by the magnitude and polarity of signal voltage. Almost all of the conventional two-valued digital circuits make use of the voltage mode and several voltage-mode multivalued logic circuits have been reported. The other category of circuit system is a current mode circuit system, in which information is expressed by the magnitude and direction of signal current. For example, an I.sup.2 L circuit belongs to the current mode circuit category and possesses the advantages of a low supply voltage, a small delay-power product and suitability for VLSI owing to the feasibility of high-density integration. The application of I.sup.2 L circuits to multivalued logic systems has been reported. For example, see "Threshold I.sup.2 L and its Application of Binary Symmetric Functions and Multivalued Logic" by T. Tich Dao in the IEEE Journal of Solid-State Circuits, vol. SC-12, No. 5, pp. 463-472 (October, 1977) and "Multivalued Integrated Injection Logic" by T. Tich Dao, Edward J. MacCluskey and Lewis K. Russell in IEEE Transactions on Computers, vol. C-26, No,. 12, pp. 1233-1241 (December, 1977).
However, since an I.sup.2 L circuit is constituted by bipolar transistors, the appearance of an error in the multi-output current mirror used in this circuit is unavoidable. The error becomes quite conspicuous particularly at saturation of one or more collectors of the multi-output current mirror. Accordingly, even though there may no problem when an I.sup.2 L circuit is applied to a two-valued logic circuit system, great difficulties are encountered when an I.sup.2 L circuit is employed in a multivalued logic circuit system of ten or more values. Further, the switching circuits used in the I.sup.2 L circuits reported so far are grounded switches. The disadvantage here is that diodes for preventing a reverse current are required when circuits which include the grounded switches, and which consume electric power regardless of whether the switches are on or off, are connected in parallel.